Restricted solid propellant



March 28, 1961 2,976,678

D. E. KENNEDY RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3Sheets-Sheet 1 E Q 4% Fig. 1

Dan 5. Kennedy V INVENTOR. 27, 2 W

ATTORNEY March 28, 1961 D. E. KENNEDY 2,976,678

RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3 Sheets-Sheet 2 Fig. 3

1 i a I i Fig. 4 I 45 Fig. 5 fi'wiffii.

r BY V6 2 D ATTORNEY March 28, 1961 D. E. KENNEDY 2,976,678

RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3 Sheets-Sheet 3nnsrnrcrnn SOLID PROPELLANT Don E. Kennedy, Park Forest, 11]., assignorto Standard Oil Company, Chicago, 11]., a corporation of Indiana FiledDec. 19, 1955, Ser. No. 553,767

1 Claim. (Cl. 650-356) This invention relates to an article ofmanufacture for the generation of a gas at high pressure. Moreparticularly the invention concerns a solid propellant grain which isrestricted with respect to burning surface whereby the gas pressuregenerated during ignition and initial burning of the grain iscontrolled.

The gas produced in the combination of grains made from propellants suchas double base powders (mixtures of nitrocellulose and nitroglycerine)ammonium perchlorate or ammonium nitrate may be used for the propulsionof rockets, the assisted-take-oif of airplanes and for the operation ofmotor starters. Normally only a part of the surface of the propellantgrain is permitted to be a burning surface. In order to obtain anequilibrium pressure in the combustion chamber a restrictor coating isplaced on a predetermined portion (S) of the surface of the propellantgrain which restrictor coating remains afiixed to some part of thesurface of the grain over essentially the entire burning time of thegrain. The restrictor coating is relatively non-fiarnmable as comparedto the flammability of the propellant portion of the grain. By acombination of surface restrictor coating and grain configuration aparticular propellant composition can be burned to provide gas in agiven chamber at a predetermined pressure i.e. following the ignitionperiod, until the propellant grain is essentially completely burned.Many materials have been used for restrictor coating such as celluloseacetate, polyvinyl acetate, polyvinyl chloride, asphalt, asbestos tape,etc.

In the ignition ofsolid propellant charge under conditions usuallyencountered in rocket, assist-take-otf and gas generator service ingeneral, it is necessary to generate and transfer heat energy to thepropellant grain surface until the ignition temperature is reached andcombustion of the propellant charge is sustained. This heat energy isprovided by an igniter which may consist of any form of combustionexposive powders such as black powder or cannon powder.

Igniters function by releasing a quantity of high temperature gases andhot particles which fiow over and give up part of their energy to thepropellant grain surface. The igniter is designed to have expendeditself shortly after ignition of the propellant grain becomes selfsustaining. During the ignition phase of the combustion of thepropellant grain the pressure level in the gas generator chamber, whichencompasses the igniter and propellant grain, is determined jointly bythe rate of gas generation from the igniter, the rate of combustion ofthe gasproducing propellant grain, and the size of nozzle orifice forescape of gas from the chamber. Upon the establishment of ignition ofthe gas-forming propellant grain and the dissipation of igniter gases,an equilibrium gas pressure is obtained which is determined by thenature and burning characteristics of the propellant material of thegrain, the burning surface area of the grain and the throat area of thenozzle orifice.

For a given propellant charge in the form of a gasforming grain andassociated chamber and nozzle, a

restrictor.

, 2,976,78 Patented Mar. 28, 1961 specific minimum igniter loading isrequired to obtain self sustaining combustion of the solid propellantgrain and thereby attain a flow of gas at equilibrium pressure. Theminimum loading varies with the temperature of the grain at the time offiring, being larger for lower temperatures than for high temperatures.It is not practicable to use variable amounts of igniter loading to meetthe ignition requirements of extremely low or extremely hightemperatures. Thus over ignition may result when igniters suitable forlow temperature firing are used for firing the same grains at hightemperature conditions; as a result ignition pressure peaks may occur inthe pressure-time trace of the firing of the grain.

The starting of turbojet engines is a difiicult feat because of thetremendous mass of metal composing the larger turbojet engines. One ofthe techniques now used for the starting of such engines involves theuse of a solid propellant driven turbine operated starter. The starteris aflixed to the frame of the engine and the turbine of the starter isconnected by clutch to the rotor of the turbojet engine. A solidpropellant grain mounted in a combustion chamber of the starter producesgas which spins the turbine of the starter and turns over the rotor ofthe turbojet engine to the necessary minimum rotational speed prior tointroduction of the fuel to the main combustors. One of the seriousproblems associated with the use of such starters lies in the peaktorque produced immediately after the igniter loading has been actuatedin the starter. This momentary torque results in excessively highmaintenance on the bearings and other moving parts of the starter.

An object of the invention is a solid propellant grain the ignition andsustained burning of which is accomplished by the use of a gas-producingigniter without producing an initial pressure during the burn-out periodof the igniter loading and the establishment of sustained burning of thesolid propellant which is greater than the equilibrium pressuremaintained in said combustion chamber during the burning of the solidpropellant grain. Another object of this invention is a solid propellantgrain which attains relatively slowly the equilibrium pressureproducable by the solid propellant grain. Still another object of theinvention is a solid propellant grain wherein the initial pressure isrelatively lower than the equilibrium'pressure produced by the burningof the solid propellant grain. A particular object of the invention is agas-generating device utilizing a solid propellant grain and agas-producing igniter which device does not sustain aninital pressureupon the firing of said igniter which is higher than the equilibriumpressure of the burning of the solid propellant grain. Other objectswill become apparent in the course of the description of the invention.

Figure l is a longitudinal view of an assist-take-ofr' unit with solidpropellant grain.

Figure 2 is a cross-sectional view of said assist-take-oif unit takenalong 22.

Figure 3 is an elevation view of a cylindrical gasproducing grainshowing short-term restriction of the grain.

Figure 4 is a cross-sectional view of Figure 3.

Figure 5 is a segmentation of Figure 3 with short-term restrictor shownin detail.

Figure 6 is a pressure-time trace illustrating the firin and burning ofa gas-propellant grain without short-term Figure 7 is a pressure-timetrace of the firing and burn- .ing of a grain with one short-termrestrictor.

polyglycols and polyglycols. ticizers for the synthetic resins areethylene glycol diglycolate, the 'acetins (mono, di and tri),triethyleneglycol 3 applied to a part of said body portion which coatingis adapted to restrain burning of the body material beneath said coatingfor a period of time at least substantially equal to the burn-out timeof an igniter adapted to establish the sustained'burning of said bodyportion and not more than about 2 seconds inexcess of said burn-outtime.

The burn-out time of the igniter is defined herein as the time intervalbetween the actuation of the element which activates the burning of theigniter loading and completion ofthe burning of said .igniter loading.In general burn-out times will range from about 20 mils to as much as 1second. A mil is defined herein as onethousandth of a second.

. The period of time that the short-term restrictor coating remains onthe surface of the body. portion of the grain, designated as theresidence time, corresponds to a period at least substantially equal tothe burn-out time of the igniter and an added period up to about 2seconds following said burn-out time. The residence time of theshort-term restrictor coating depends on several factors. Some of thesefactors are combustibility, heat cnductivity and thickness of thecoating as affecting the overall rate of heat transfer through thecoating material. Other factors are ignitibility of the body materialbeneath the short-term restrictor coating as Well as the temperature ofthe grain at the time of firing. The material is preferably combustibleunder the temperature conditions existent in the combustion chamber,that is the motor, during the burn-out time of the igniter.

. Materials which may be used for short-term restriction are adhesivetapes consisting of paper, cloth or plastic provided with adhesive toinsure intimate contacting of the tape with the body portion. Thincoatings of asphalt and coatings of liquid self-setting plastics may beapplied in the form of solutions thereof or a solution ofpolyisobutylene may be applied as a short-term restrictor coating. f

The area subjected to short-term restriction determines the effectiveremaining burning surface of the grain dur- "ing the ignition periodthereof and hence controls the of the short-term restriction time wasabout 1 seconds The short-term restrictor coatings of this invention.are particularly adaptable to ammonium nitrate based gasformingpropellant grains. These grains usually comprise at least about 67%ammonium nitrate, from about 10% to about 25% binder material and from1% to about 10%,. combustion catalyst. The binder material isthermoplastic and consists of about 18% to about 50% of a plasticizablesynthetic polymeric material, thatis, a

thermoplastic synthetic resinand from about 50% to about 82% of at leastone plasticizer for the synthetic resin. Examples of synthetic resinswhich may be used are cellulose acetate, cellulose acetatebutyrate,-polyvinyl' acetate and polyvinyl chloride. The plasticizersare oxidizable andpreferably oxygen-containing and maybe"classifiedbroadly'as polymeric esters, esters of polyhydric alcohols,ethers of nitrophenols, nitromonocyclic aromatics, esters ofpolycarboxylic acids, alkyl ethers of Specific examples ofplasdi-Z-ethylbutyrate, triethylene glycol. .di-2-ethylhexoate,

polyethylene glycoldi-Z-ethylhxoate, triethyl citrate, acetyl triethylcitrate, dimethyl ph'thalate, dioctyl phthalate',,nitromethylpropanedi'ol diacetate, dinitrophenyl thickness.

awasrs propyl ,ether, dinitrophenyl allyl ether, nitrodiphenyl ethers,dinitrophenoxyethanol, bis(dinitrophenoxy)ethane, dinitrotoluene,triethylene glycol, and polyethylene glycol. Combustion catalysts whichmay be used in the ammonium nitrate based grains are the Prussian blues(soluble and insoluble), the alkali metal chromates, alkali metaldichromates and ammonium chromate or ammonium dichromate or organiccombustion catalysts such as the mono sodium salt of barbituric acid,which is particularly suitable as catalyst-for grains used in gasturbine starting service. Other components such as asphalt and carbonmay be added to the ammonium nitrate based grain to improve coldtemperature ignition of the grain. Amines may be added to chemicallystabilize the grain against decomposition in hot storage.

The ammonium nitrate based gas-producing propellant compositons may bemolded or extruded into grains. The molds may be provided with insets toprovide longitudinal apertures in the grains. These may be cruciform,circular, star-shaped or any other desirable shape in cross section toprovide for internal burning of the grain.

An illustrative double base gas-forming propellant which can be castinto grains and restricted with shortterm restrictors comprises, on aweight basis, about to of a mixture of nitrocellulose andnitroglycerine, about 10% to 15% of a mixture of triacetin and dioctylphthalate, about 3% of leadsoap combustion rate modifier and about 1 to2% of an amine such as 2-nitrodi phenylamine. Likewise the short-termrestrictor coating can be applied to ammonium perchlorate based grainsfor use in assist-take-otfservice and for rocket propulsion. Arepresentative composition of such body material comprises on a weightbasis about 70% to 75% ammonium perchlorate, about 25% of a bindermaterial consisting of a polyester resinstyrene copolymer and less than0.5% of copper chromite catalyst.

Figure 3 is an illustration of a cylindrical gas-producing grain havingbody portion 41 which has centrally located longitudinal cylindricalaperture 42 extending therethrough. The annular ends of body portion 41,are restricted with restrictors 43 and 44. The permanent 'restrictorsmay consist of plates of synthetic resin such burned. A

Body portion 41 is also provided with short-term restrictor coating 45which may consist of a pressure sensitive tapewhich suitably'may be ofabout 0.006 inch Short-term restrictor 45 encircles the exteriorcylindrical surface of body portion 41 and adheres thereto adjacentpermanent restrictor coating 44 for a predetermined period, such as 2seconds, following the ignition of the igniter charge therebydelayingignition and combustion of the part of thecylindrical surfacebeneath tape coating 45 .over that period when gases from the ignitedigniter are effective in producing over-pressure and over the periodrequiredjto establish sustained burning of the body material. 7 Y

3 Figure. 4. is a cross section "of the grain of Figures showinginternal aperture .42, permanent restrictor coatings14 3 and-44 andshort-term restrictor 45 and :Figure 5, is a segmentation of the grainof Figure ,3 showing. permanent restrictorxcoating 44' and 'short termrestrictor coating 45.

. 1a Figure. 1, the fbody of an arc unit is meanest atubuiar member 11'which is closed at one end and which is provided with threads at theopen end. Member 11 is provided with two loops, 12 and 13. These loopsare used to hang the unit from a carrier, not shown, which is attachedto the wing of the aircraft. This carrier makes it possible to jettisonthe unit after take-off. A funnel shaped member 14 is attached to member11 by engagement of the threads at the large open end of member 14 withthe threads of member 11. Member 14 is provided with a nozzle 16 throughwhich the decomposition products pass. The size of nozzle 16 determinesin part the pressure maintained inside the chamber formed by members 11and 14.

Solid propellant fills the cylindrical portion of member 11. The solidpropellant of this illustration consists of seven tubular grains, 17,18, 19, 20, 21, 22 and 23; each having an CD. of about 3 inches andhaving a centrally located cylindrical opening 1 inch in diameter thefull length of the grains which are approximately 30 inches long. Thegrains consist essentially of combustion catalyst, oxidizable binder andammonium nitrate.

Each grain has the annular end areas 24, 25, 26, 24a, 25a and 26arestricted against burning with permanent restrictor coating consistingof asphalt or other suitable material such as cellulose acetate in orderto limit burning to the cylindrical surfaces. Each grain is restrictedwith combustible tape 27, 28 and 29 applied to the external cylindricalsurfaces near the ends of the grains most remote from the ignitercharge, that is, the firing end of the chamber. The thickness of thistape restrictor may be varied to obtain the desired time of short-termrestriction and the width of the tape restrictor may be varied to obtainthe desired amount of pressure reduction over the period of ignition andto establish sustained burning of the grain.

Although a multiplicity of separate tubular grains encased in the singlecombustion chamber are illustrated herein, the invention is not limitedto such shaped grains or to use of multiple grains in the combustionchamber. Any particular shape may be utilized. Examples of other shapesare cylinder, cruciform, triform, hexoform, octaform and slab. Whereperforated grains are employed the longitudinal perforation may becircular or star-shaped with various numbers of points in the star.Furthermore, a single cylindrical grain having a single longitudinalperforation or multiple longitudinal perforations may b used in thecombustion chamber.

The grains are held in position and prevented from sliding back andforth in the chamber by means of a wire grid 30. Wire grid 30 consistsof a ring cut to fit the threads of member 14 and is provided with agrid of metal wires which resist the high temperature existing in thecombustion chamber.

An igniter means is positioned within member 14 so as to close off thenozzle 16. The igniter means consists of a container 31 filled withblack powder or cannon powder or cannon powder or other easily ignitedmaterial, which upon ignition and burning produces a large volume of gasat elevated pressure.

Mixtures of the different grades of black powder may be used to adjustthe overall burning rate of the powder. The igniter for commercial sizeassist-take-olf grains may consists of the C grade cannon powdergranules or mixture of the C grade with black powder having an averagediameter less than the C grade. The amount of igniter is determined bythe surface area to be ignited, freevolume space in the combustionchamber, the ease of igniting as determined by the surface andcomposition of the grain and temperature of the grain when fired. Asquib 32 for igniting the powder, is attached to the container 31 incommunication with the powder contained therein. Electrical wires 33connect a wire in the squib to the electrical system of the aircraft anda switch therein.

On one side of the conical portion of member 14 a safety venting means34 is provided for the combustion chamber. Venting means 34 comprises atubular member fastened to member 14, which tubular member has fullaccess to the combustion chamber and is provided with a rupture disc,not shown. The rupture disc is of such construction that excess pressurein the combustion chamber will blow out the disc whereby damage to theunit resulting from over-pressure in the combustion chamber is avoided.

The assist-take-off unit is assembled as follows: Grains are insertedinto member 11. Venting means 34 is attached to member 14. Igniter 31 isinserted through the large open end and fitted so as to close thenozzle, wires 33 having first been passed through nozzle 16. Wire grid30 is screwed into the large open end of member 14 and the assemblednozzle portion is then screwed onto member 11. The assembled unit isthen attached to the wing of the aircraft by loops 12 and 13 and wires33 are connected to the electrical operating assembly in the aircraft.When the pilot desires to obtain the assist-take-off, he throws theswitch which causes a current in wire 33 and heats the firing wire insquib 32 which in turn ignites the powder in container 31.

Container 31 is of suflicient strength to withstand the initial pressuregenerated by the gases from the powder. However, the hot igniter gasraises the pressure and causes the pressure to rise to a point whichcannot be resisted by container 31. Container 31 disintegrates and thefragments are discharged through nozzle 16.

As the gases pass out of the nozzle the reaction acts on the aircraftand adds to the thrust to assist the aircraft propellers and a markedincrease in forward speed results. A shorter space of time for take-offand/ or a heavier load can become airborne than when propellers only areused.

Tests A series of ammonium nitrate based turbojet auxiliary turbinestarter grains was prepared. These grains were in the form of cylindersapproximately 4 inches in length and 5 inches OD. and were provided withcentrally located longitudinal cylindrical apertures having a diameterof 1.5 inches. Both annular ends of the grains were restricted withpermanent restrictor disc plates about 4 mm. in thickness. These discplates consisted of plasticized cellulose acetate. The composition,method of preparing the composition and method of shaping thecomposition into permanently restricted propellant grains is describedon pages 11 and 12 of the copending application of William G. Stanleyentitled Restricted Solid Propellant Grain, Serial No. 549,275, filedNovember 28, 1955.

To one of the prepared grains was applied a single layer coating ofshort-term restrictor consisting of pressure-sensitive fiat-back papertape having a width of inch and a thickness of 0.006 inch, the tapebeing positioned at one end of the grain immediately adjacent theperiphery of the permanent restrictor disc around the externalcylindrical surface of the grain. A second grain of the above series ofgrains was provided with short-term restriction I by applying two layersof the above paper tape adjacent the permanent restrictor disc thusproviding this grain with a shortterm restrictor of inch width andapproximately 0.012 inch thickness around the external cylindricalsurface. A third grain of the above series was used as a control grain,that is, the grain was provided only with permanent annular endrestrictor discs and not with short-term restrictor tape coating.

The above grains were fired separately at 70 F. grain temperature in atest motor having a gas eiilux diameter of 0.236 inch using the sameamounts of the same igniter charge to ignite the grains. The grains,provided with short-term restrictor, were positioned in the motor withthe tape-restricted section being at the end thereof most.

V is usually the time interval between about 0.2 and 0.3 secgrainprovided with a single layer of the paper backed tape and Figure 8 isthe trace of the grain having double thickness of the tape appliedthereto. Pressures developed are indicated on the vertical axes and timeintervals are indicated on the horizontal axes. The legend at the top ofFigure 6 indicates the time interval of 3 seconds for 15 divisions, thatis 0.2 second for each division which scale is the same for Figures 7and 8.

The trace of Figure 6 shows a maximum initial pressure of about 1360p.s.i.g. with pressure of about 1320 p.s.i.g. in critical period of0.2-0.3 second after firing of the igniter charge... 1

Comparing the trace of the firing of single thickness tape restrictedgrain, Figure 7, with the trace of Figure 6, the maximum pressureoccurring in the 0.2-0.3 second critcial period was 1000 p.s.i.g. Theresidence time of the short-term restrictor in the firing of the grainwas about one second.

The effect of increasingthe thickness of the short-term restrictor isshown in Figure 8. The maximum pressure over the 0.2-0.3 second criticalperiod was about 1080 p.s.i.g. Doubling the thickness of short-termrestrictor applied increased the residence time to about 1.6 seconds,

3 Having thus described my invention, I claim:

An apparatus adapted for the production of high pressure gas from theburning of a solid propellant which comprises a chamber provided with agasdischarge'opening and positioned within said chamber, an igniterassembly containing a gas-producing material the burning of whichaffords a sustained burning of a solid propellant grain positionedwithin said chamber, said assembly containing an igniter means foractuating the burning of said gas-producing material, wherein said solidpropellant com- I prises a tubular body portion comprising ammoniumnitrate, oxidizable thermoplastic binder and a combustion catalyst,permanent restrictor coatings applied to the annular ends of saidtubular body portion which remain on the unburned surface of the grainuntil that surface is substantially burned away and a short-termrestrictor coating applied to the external cylindrical surface of saidbody portion immediately adjacent the periphery of one of said permanentrestrictor coating which short-term restrictor coating is adapted torestrain burning of the body material beneath said short-term coatingfor a time at least substantially equal to the burn-out time of saidg'asproducing material and not more than 2 seconds in excess of saidburn-out time and wherein said short-te1m restrictor coating consists ofa combustible pressure sensitive tape having a thickness of about 0.006inch.

References Cited in the file of this patent UNITED STATES PATENTS2,366,165; Whitworth et a1 Jan. 2, 1945 2,464,181 -Lauritsen Mar. 8,1949 2,484,355- -Parsons Oct. 11, 1949 2,539,404 Crutchfield Jan. 30,1951 2,574,479 Hickman ..'Nov. 13, 1951 2,643,611 Ball June 30, 1953FOREIGN PATENTS Great Britain Aug. 15, 1949

